An analytical sieve apparatus can make use of set of sieves below which is provided a sieve tray forming part of the stack. A particulate product to be subjected to particle-size analysis is deposited upon the upper sieve of the stack and the stack is agitated to cause particulate material to pass downwardly through the openings in the screening layers spanning the annular frames of the respective sieves. These openings are graded in steps to be progressively small downwardly, whereby only the material passing the smallest sieve will collect in the tray while, on each sieve above the tray, only the material passing the preceding sieve but not passing that sieve will collect.
An analytical sieve apparatus of that type is described in the brochure of Rhewum entitled "Analysen-Siebgerate" Liste 2000, 12.86.
Sieve analyses are important over the entire range of particle-size measurement technology. They are used to determine particle-size distribution of flowable solid products, for example, dust, meal, powder or granulates and depending upon the product, can be carried out in a dry or wet phase.
The opening sizes of the sieves are graded downwardly in the stack in accordance with a particular standard. For example, the major widths of the sieves may be graded from top to bottom decreasingly in accordance with the German Industrial Standard DIN 4188.
The stack of sieves can be agitated by hand or by an agitator of a testing machine.
After termination of the sieving time, usually also established by a particular standard, e.g. the German Industrial Standard DIN 66 165, the residues on the sieves of the stack and the material collected in the tray are collected from the respective surfaces, by brushes or the like and swept into respective containers and weighted. The weights of the individual residues and the weight of the particular material passing through the finest sieve are calculated in proportion to the total weight of the starting sample and the resulting particle-size distribution is presented either as a table or a curve on an appropriate grid or histogram.
It has been found that the removal of the residues after the sifting process as determined has posed a problem with conventional systems, especially where there was a danger of incomplete removal from very fine sieve fabrics, and with the precise weighing of the respective portions.
In addition, both the removal of the portions of the particles on the sieve surfaces and in the tray and the weighting process are labor intensive procedures which are susceptible to variation depending upon the personnel carrying them out. This, of course, can influence the weighting percent or mass percent of a particular component and thus the graphic display of the particle-size distribution or the histogram.
Especially in quality-control operations of particle-size distribution measurements there may be a problem since such systems require complete exclusion of subjective actions by the quality-control personnel.
Furthermore, utilizing conventional systems, a sieve analysis may take anywhere between 15 and 45 minutes, depending upon the material, the fineness, the sieving time and the type of analysis presentation which is desired.